Corrosion-resisting steel

ABSTRACT

THE CORROSION-RESISTING STEEL WITH SUPERB RESISTANCE TO PITTING CORROSION IS MANUFACTURED OF A MICROSTRUCTURAL STAINLESS STEEL COMPOSED MAINLY OF CHROMIUM OR CHROMIUM ADDED WITH AT LEAST ONE OF NICKEL AND MANGANESE STILL CONTAINING 10 TO 30 PERCENT BY WEIGHT OF CHRONIUM AND FURTHUR HAVING 2 TO 10 PERCENT BY WEIGHT OF ONE OR MORE CARBOONITRIDES SELECTED FROM THE GROUP OF TITANIUM, ZIRCONIUM, NIOBIUM, TANTALUM AND VANADIUUM UNIFORMLY AND FINELY DISPERSED THEREIN.

y 28, 1 YOSHIHIKO ABE 3,813,240

' CORROSION-RESISTING swam.

Filed June 21, 1972 2 Sheets-Sheet 1 E 20- o I I Q 2 A HYDROCHLORIC ACID 5 SOLUTION F' l5 (ROOM TEMPERATURE) Z 9 g a: 5 CC 0 O I I l O i 2 4 5 QUANTITY OF MOLYBDENUM(%) 5 250- v 35%HYDROCHLORIC ACID g, SOLUTION I E (ROOM TEMPERATURE) I 1 I50- 0 CE I00- 0 l r O i 2 3 4 5 QUANTITY OF MOLYBDENUM (-%I fi IE 250-: Ar, 3 I 5% SULFURIC ACID SOLUTION 0: 2 (BOILING) g No.I? o No.20 8 X N0.,I8 No.21 E. No.19 A No.22 U

' 400 560 660 s' oo 9'00 I600 II'oo 3 7 -HEAT TREATMENT TEMPERATURE (C)- May 28 1974 Filed June 21, 1972 FIG. 2

FIG. 2A,

FIG.

-l QUANTITY OF COPPER( CORROSION RATE (g/m /hr) CORROSION RATE (g/m /hr YOSHIHIKO ABE 3,813,240

CORRQSIQN'RESISTING STEEL 2 Sheets-Sheet 2 33 NITRIC ACID SOLUTION (BOILING) 0.6-

18% CHROMIUM STEEL 0.2-

0 012 014 0. 6 O'a ,Ifo

QUANTITY OF ALUMINIUM(%) 65% NITRIC ACID SOLUTION (BOILING) O 25% CHROMIUM STEEL o d2 o.'4 016 018 1:0

QUANTITY OF ALUMINIUM l I DOMAIN OF SUPERIOR I CORROSION RESISTANCE l 1.0-

XI 0 A DOMAIN OF GOOD I CORROSION RESISTANCE 0.5 \O o o R X X X DOMAIN OF INFERIOR CORROSION RESISTANCE 0 OS L0 L5 QUANTITYOF TITANIUM(%') United States Patent 3,813,240 CORROSION-RESISTING STEEL Yoshihiko Abe, Tokyo, Japan, assignor to Mitsubishi Seiko Kabushiki Kaisha, Tokyo, Japan ing specific carbonitrides dispersed therein uniformly and finely beyond a certain degree of concentration.

BRIEF DESCRIPTION OF THE DRAWINGS These and other objects of the present invention will Filed June 21, 1972, Ser. No. 264,985 5 become more apparent to those skilled in the art when Claims priority, application Japan, Mar. 3, 1972, considered in reference to the following detailed descrip- 47/21,554 tion in the light of the acompanying drawings wherein: f- (3 C 37/10. 39/14 FIGS. 1 and 1(a) show the result of tests, in which the US. (1 75- 6 D 2 Claims efi'ect of molybdenum quantity contained in the steel ac- 10 cording to the present invention was tested in 2% and ABSTRACT OF THE DISCLOSURE 35% hydrochloric acid solutions;

FIGS. 2 and 2(a) show the results of tests, in which the P T' steel superb ,Teslstance 1 effect of aluminum quantity contained in the steel accordplthng 1S manufactmled of a mlcmstfucmra ing to the present invention was tested in 33% and 65% stainless steel composed mainly of chromium or 5 nitric acid Soluions; chfomlum aflded f at least of mcke} and FIG. 3 shows the test result of six samples in Table 2 manganese still containing 10 to 30 percent by weight of comparing their corrosion resistances under several chromium and further having 2 to 10 percent by weigh; conditions of heat treatment; and h or lhore p z selected from the 0 FIG. 4 shows a quantitative relation of copper and tlthmum, Zlrcomum, P tantalflm and vanadmm titanium causing an improvement on corrosion resistance uniformly and finely dispersed therein. in the test shown in DETAILED DESCRIPTION OF INVENTION BACKGROUND OF TH INVENTION The steel according to the present invention is produced The present invention relates to corrosion-resisting by a Promss whlch the mcflten steel much steels, and more particularly to steels with superb excess carbon compared W1 th Oxygen, hlh'ogell and resistance to pitting corrosion and excellent corrosion cfmtents are added some elementF to form resistance against any kind of acid solution. carbonitrides so that these elements preferentially react In general, corrosion resistance is an indespensahle with carbon, and the thus-obtained solid carbides are unicharacteristic for steels to be used in corrosive a a dlspersed Preclpltated 95 wlth the sohd circumstances and it is well known that many kinds of nitrides and the dispersed carbonitrides must be stable stainless steel have so far been developed for use under even under hot and 9F conditions thereafter such circumstances. However, such stainless steels have elements to Preclpltate such f f l dlspersold neither sufficient pitting corrosion resistance cop partlcle and to produce stable carbomtride s in a hot and rosion resistance equally eifective against all acid solucorroslve envlmnmfmt tltalllum, zlrcomum mobmm ,dons so the steels have to be selected acording to the tantalum and vanadium are given. One or more elements corrosive environment under Which they are used. of them constitute such sole or complex carbonitrides. In 7 these carbonitride dispersion-strengthened materials, size SUMMARY OF THE INVENTION of dispersoid particles, homogeneity and dispersed uantit are the most im ortant factors to atiect the A principal object of the present invention 1s to provide gharacgristich p corrosion-resisting steels without the drawbacks as in- It was f d out that adding 02% or less phosphorus hefent as 111 h cmlventlonal steifls as Y is effective to improve the homogeneity and preferential Allqlher P1? of the P F Invention 1S Provlde forming of boronnitride in the molten steel by adding corrosion-resisting steels which have a superb pitting coror less boron is desirab1e rosion resistance quality as unobtamable 1n the conven- From the foregoing explanation, it is evident that the B01181 s I steel according to the present invention can be produced A h Phl 0f the Ph mventlon Provide industrially. The following explanation relates to the kinds collfloswn-reslshng Steels Whlch have a matrix of cheap and necessary quantity of carbonitrides evenly dispersed ferrite structure. i into the steel according to the present invention, and also St1 l furthe1 J the h lnvehtlon 15 to components and necessary quantity of the matrix in which provide COIKPSIOII-IESlSUHg Steels which show excellent cal-bonitrides are dispersed corrosion resistance to various acid solutions. The result of the tests having investigated the effect of A steel according to the present invention is characterquantity of dispersed carbonitride and quantity of ized by the fact that it is manufactured by adding into a chrominum in the matrix for pitting corrosion in 5% ferconventional stainless steel some alloy components havricchloride solution (pH 1.25) is shown in Table 1.

TABLE 1 S 1 Dispersed carbonitride Components in matrix (percent) am 8 num er Element(s) Percent Cr Structure Corrosion appearance 5.1 5.08 Cr ferrite Teihribllsr uniform 8 ac o 4.7 8.7 d D 3.2 10. 4a d Fitting. 6.1 13.1 Cr-Mo ferrite" Do. 5.4 15.6 Cr ferrite Do. 7.5 17.6 Cr-Mn austenite- Do. 5.0 18.3 Gr-Ni austenite.. Not pitting 3.9 17.0 Cr-Ni-Mn austeni e ting. 3.5 17.9 Cr-Mo ferrite Not pitting. 2.3 17.2 .do Do. 1.4 18. 4 Localized pitting. 0 17.9 Uniform attack. 0 18. 7 Localized pitting. t: cr-hlli rti'grrite-aut Not attack.

6 4.7 24.5 G r-Ni aiistemt D8. 3.9 25.1 Or-Mn austenite..- Do.

Table 1 shows that it is necessary to add or more chromium into the matrix to completely prevent corrosion of the steel regardless the micro-structure of the matrix.

Furthermore it is comprehensible that any kind of carbonitride increases its resistance to pitting corrosion, if 2.0% or more of sole carbonitride of titanium or zirconium, or complex carbonitride of titanium and niobium, titanium and vanadium, or zirconium and tantalum are dispersed into the matrix containing 10% or more chromium. Therefore, in the steel according to the present invention, it is inevitable to precipitate dispersely 2% or more carbonitride abovementioned into the steel.

If the matrix of steel contains 10% or more chromium, any kind of existing stainless steels can be applied, whether the structure of matrix is ferrite containing chromium as 4, The corrosion resistance of the steel of the present invention to sulfuric acid can be remarkably increased by containing three elements, nickel, copper and titanium together in the matrix.

FIG. 3 represents a comparison of corrosion resistances to 5% sulfuric acid solution of six samples having different quantities of nickel, copper and titanium in the matrix as shown in Table 2. It is found out that adding only two elements combined, copper and nickel, copper and titanium or nickel and titanium does not show marked effect to corrosion resistance, but the ferritic steel containing three elements, nickel, copper and titanium, together has excellent corrosion resistance under the condition of 500-800 C. heat treatment and the austenitic steel containing three elements above mentioned, has also strong corrosion resistance under any condition of heat treatment.

TAB LE 2 Dispersed carbonitrlde Components in matrlx, percent a main component of alloy, or ferrite-austenite or austenite containing one or two elements of chromium, nickel and manganese. In the steel according to the present invention, however, maximum quantities of dispersed carbonitride and chromium are limited from the production engineering standpoint, and it is desired that quantity of carbonitride is limited to 10% or less owing to fluidity and quantity of chromium is limited to or less due to plasticity.

The corrosion resistance of the steel according to the present invention to acid solution containing chloridion can be remarkably improved by containing molybdenum in the matrix.

FIGS. 1 and 1(a) shows the variation of corrosion rate of the steel according to the present invention in hydrochloric acid solutions of difiFerent concentrations against the quantitative change of molybdenum content in the matrix. The effect of molybdenum appears conspicuously about 0.7% and saturates about 4% Therefore quantity of molybdenum content may be limited to 05-50%.

It is remarked additionally that the etfect of added molybdenum is not influenced by quantity of chromium or carbonitride as far as the steel of the present invention is cornerned.

Similarly the corrosion resistance of the steel according to the present invention to nitric acid can be improved by adding aluminum in the matrix. FIGS. 2 and 2(a) represent the variation of corrsion rate of the steel against quantities of aluminum in its matrix containing 18% and 25% chromium respectively, and having 3.55.0% various carbonitrides dispersed.

In both cases, it is found out that the effect appears at 0.2% or more and saturates at 0.5% or more.

Hence, considering the defect of aluminum for plastic deformation, quantity of aluminum may be limited to 0.2l.0% in maintaining its efiect sutficiently.

In the steel containing three elements, nickel, copper and titanium, together, quantity of nickel shall be preferably more than 1.5 times that of copper to prevent hot brittleness due to copper. FIG. 4 shows the domain of corrosion resistance rate of the steel against various quantities of copper and titanium, keeping quantity of nickel content 1.5 times that of copper. Minimum quantities to improve the corrosion resistance are 0.4% copper and 0.2% titanium.

It has been made clear that the corrosion resistance is improved as added elements increase, and it is much improved with 0.9% copper and 0.6% titanium added, but the resistant property saturates over that quantity.

Therefore, for corrosion-resisting steel to sulfuric acid, ferrite-austenitic steel and austenitic steel containing 10- 30% chromium and at least 3% nickel, shall have 2-10% said carbonitride dispersed into the matrix containing 0.4- 20% copper and 0.21.5% titanium, and other stainless steel shall have the same quantity of carbonitride dispersed into the matrix containing 0.63.0% nickel additionally. In case that the corrosion resistance to hydrochloric acid or nitric acid is to be supplemented, one or two elements of 0.5-5.0% molybdenum and 0.21.0% aluminum shall be added.

The following examples will serve to illustrate the excellent corrosion resistance of the steel according to the present invention.

EXAMPLE 1 SUS24B containing 16-18% chromium as a main component and the steel according to the present invention in which 3.2% by weight of complex carbonitride of titanium and niobium was dispersed, were immerged in acid solution containing 5.0% chloridion. Pitting corrosion appeared on SUS24B in three days, but no corrosion was found on the invented steel after ten days.

Test pieces of 25 x x 50 mm. were cut oif from the two steels and they were immerged in boiling solution of sulfuric acid containing 10% copper sulphide for one hour.

Then SUS24B got terrible corrosion over the entire surface and 40% of it was dissolved, while the invented steel had no remarkable corrosion and dissolution rate was less than 0.1%.

EXAMPLE 2 SUS42B, which contains 24-26% chromium, 19-22% nickel as main alloy components, is the highest grade stainless steel having excellent corrosion resistance.

The steel according to the present invention which has carbonitride containing 4.0% titanium carbide as a main component, dispersed in the matrix having only 2% molybdenum besides the same quantity of chromium as SUS42B, was tested for corrosion resistance comparing with SUS42B in acid solution containing chloridion.

As shown in Table 3, the invented steel proved to have extraordinarily excellent corrosion resistance judging from both corrosion appearance and corrosion rate, and the difference was obvious in sodium chloride solution.

TABLE 3 Corrosion rate gJmfl/day (corrosion appearance) Corrosive solution EXAMPLE 3 The steel according to the present invention which has carbonitride containing 3.7% titanium carbide as a main component, dispersed in the matrix containing 18% chromium, 1.2% nickel, 0.75% copper, 0.25% titanium, 2.0% molybdenum, was heat-treated at 650 C. and tested for corrosion resistance in various acid solutions comparing with 18% chromium, 8% nickel stainless steel (SUS27B) most commonly used.

The test result shown in Table 4 reveals that the steel of the present invention is slightly inferior to SUS27B in corrosion resistance to nitric acid, but it is superior in corrosion resistance to sulfuric acid, and moreover it has specific high corrosion resistance in acid solution containing hydrochloric acid and chloridion.

TABLE 4 Corrosion rate (g./m. /hr.)

Invented Invented steel (no steel (0.4% aluminum aluminum Corrosive solution contained) contained) SUS27B 2% hydrochloric acid solution (room temperatur 0. 87 0. 2. 2 2% hydrochloric acid solution oiling) 31. 2 31. 5 54. 3 2% hydrochloric acid solution (boiling 780 774 8, 180 36% hydrochloric acid solution (room temperature) 29. 9 30. 7 429 33% nitric acid solution oiling) 0. 14 0. 08 0. 07 65% nitric acid solution (boilin 0. 82 0. 22 0. 22 5% sulfuric acid solution 0 'n 23 20.5 179 sulfuric acid solution (40 C.) 0.17 0. 15 0. 56 6.1% NaCl solution (room temperature) 3.3Xl0- 3.3)(10 360x10 Even in this case, the steel according to the present invention, if 0.4% aluminum added, has the same corrosion resistance to nitric acid as SUS27B.

What is claimed is:

1. A corrosion-resistant steel having a microstructure consisting essentially of 2-10% by weight of a carbonitride of one or more elements selected from the group consisting of titanium, zirconium, niobium, tantalum and vanadium uniformly and finely dispersed in a stainless steel consisting essentially of 10-30% of chromium, an eflective amount up to 0.2% of phosphorus, an efiective amount up to 0.2% boron and the balance being iron.

2. A corrosion resisting steel having a microstructure consisting essentially of 2-l0% by weight of a carbonitride of one or more elements of titanium, zirconium, niobium, tantalum and vanadium uniformly and finely dispersed into the stainless steel, 10-30% by weight of chromium, one or more elements of less than 0.2% boron or phosphorous, 0.5-5.0% by weight of molybdenum, 0.2l.0% by weight of aluminum','0.4-2.0% by weight of copper, O.6-3.0% by weight of nickel, 0.21.5% by weight of titanium, and the balance being iron.

References Cited UNITED STATES PATENTS 3,690,869 9/ 1972 Zelenodolskaya 75-124 3,152,934 10/1964 Lula 75-l24 3,365,343 1/1968 Vordahl 75-124 3,674,469 7/ 1972 Stewart 75124 HYLAN-D BIZOT, Primary Examiner US. Cl. X.R. 

